Allan MacGregor has posted his latest in his look at design patterns in PHP with this most recent post about the Singleton pattern.
The singleton pattern is used to restrict the instantiation of a class to a single object, which can be useful when only one object is required across the system. Singletons are designed to ensure there is a single (hence the name singleton) class instance and that is global point of access for it, along with this single instance we have global access and lazy initialization.
He provides a basic Singleton implementation in PHP, a "User" class that always returns the same instance of itself no matter how many times the "singleton" method is called. He continues on and touches on one of the pain points around singleton use - many developers consider them an anti-pattern because their results can make it difficult to correctly test. He talks about how they break the Single Responsibility Principle (part of SOLID) and how they can hide dependency injection.
Singletons, Anti-patterns, and patterns in general are not good or bad; what makes a Singleton an Anti-pattern is not the pattern itself but how often is poorly implemented and how easy it is to do so.